a line, produces no light. For a visible ray of light, however narrow
it may be, has always some width, and consequently it is necessary, in
representing the wave whose progression constitutes the ray, to put
instead of a line AC some plane figure such as the circle HC in the
following figure, by supposing, as we have done, the luminous point to
be infinitely distant. Now it is easy to see, following the preceding
demonstration, that each small piece of this wave HC having arrived at
the plane AB, and there generating each one its particular wave, these
will all have, when C arrives at B, a common plane which will touch
them, namely a circle BN similar to CH; and this will be intersected
at its middle and at right angles by the same plane which likewise
intersects the circle CH and the ellipse AB.

[Illustration]

One sees also that the said spheres of the partial waves cannot have
any common tangent plane other than the circle BN; so that it will be
this plane where there will be more reflected movement than anywhere
else, and which will therefore carry on the light in continuance from
the wave CH.

I have also stated in the preceding demonstration that the movement of
the piece A of the incident wave is not able to communicate itself
beyond the plane AB, or at least not wholly. Whence it is to be
remarked that though the movement of the ethereal matter might
communicate itself partly to that of the reflecting body, this could
in nothing alter the velocity of progression of the waves, on which
the angle of reflexion depends. For a slight percussion ought to
generate waves as rapid as strong percussion in the same matter. This
comes about from the property of bodies which act as springs, of which